This invention relates to a semiconductor substrate, and a method of treating a semiconductor substrate.
Nowadays, a large scale integrated circuit (LSI) has been extensively utilized in the important portion of a computer and of a communication device. A monocrystalline silicon substrate (or silicon wafer) to be employed in the preparation of the LSI can be produced by means of Czochralski method (CZ method) for instance.
The silicon substrate prepared by means of this CZ method (a CZ silicon substrate) is however accompanied with a problem that a large number of oxygen precipitates (Bulk Micro Defect: BMD) are caused to be formed on the surface or interior of the silicon substrate. It is believed that the formation of BMD is brought about by the oxygen which is eluted at first from a quartz crucible in the process of pulling up a silicon monocrystal and, after being super-saturated in the silicon substrate, precipitated in the silicon substrate during the heat-treatment process of the silicon substrate. Because of this, the BMD is assumed to be formed of a compound consisted of oxygen and silicon (i.e. SiOx). Furthermore, it is assumed that there are two cases in the precipitation state of the BMD, i.e. one of the cases is that the BMD is precipitated on the inner surface of the voids (hole) of the silicon substrate, and the other is that the BMD is precipitated to completely fill the voids of the silicon substrate.
It is reported that the reliability of a thermal oxide film that will be formed by the thermal oxidation of the surface of a silicon substrate has something to do with the density of BMD on the surface portion of the silicon substrate. Specifically, when a silicon substrate having a high BMD density at the surface portion thereof is employed, a breakdown voltage of a capacitor gate oxide film will be decreased (random failure).
In order to remedy the failure in breakdown voltage of this kind, the following two countermeasures have been conventionally adopted.
Namely, one of such countermeasures is to anneal the silicon substrate at a high temperature of about 1,200.degree. C. in an inert gas atmosphere such as hydrogen gas or argon gas. When the silicon substrate is annealed at such a high temperature, the oxygen existing at the surface portion of the silicon substrate is diffused out of the silicon substrate and at the same time the BMD in the silicon monocrystal is re-solubilized as a solid-solution at a surface region, i.e. extending from the surface of the silicon substrate to a depth of about 50 .mu.m, thereby forming a defect-free layer, or a layer of DZ (Denuded Zone).
If a silicon oxide film is formed through a thermal oxidation on the surface of this silicon substrate provided with such a DZ layer (hereinafter referred to as a DZ silicon substrate), the percent defective in breakdown voltage can be extremely reduced.
The other of such countermeasures is to employ a silicon substrate on which an epitaxial silicon film is formed in advance (hereinafter referred to as an epitaxial silicon substrate). Since oxygen is not intentionally added to this epitaxial silicon substrate in general, the epitaxial silicon film constituting the surface of the substrate is free from the formation of BMD.
Therefore, when a silicon oxide film is formed through a thermal oxidation on the surface of this epitaxial silicon substrate, the percent defective in breakdown voltage can be extremely reduced as compared with an untreated CZ silicon substrate as in the case where a silicon oxide film is formed on a DZ silicon substrate.
However, even if it may be possible to remedy the defect in breakdown voltage of the oxide film, the wear-out life (intrinsic failure) of the oxide film can not be improved, but would be left remained at substantially the same level as that of an untreated CZ silicon substrate.
Because of these reasons, the silicon oxide film formed on the CZ silicon substrate treated in advance with any of the aforementioned countermeasures is still accompanied with a problem that it is insufficient for use as a tunnel gate oxide film for an EEPROM, since the tunnel gate oxide film would be exposed to a Fowler-Nordheim type current stress of high electric field and therefore is desired to have a long dielectric breakdown life.
As explained above, the employment of a conventional silicon substrate which is provided with the DZ layer or with an epitaxial layer is accompanied with the problem that even if the random failure of a silicon oxide film may be overcome, the intrinsic failure cannot be overcome.